There are different known approaches to accomplish wavelength tuning and/or frequency modulation (FM) using an electro optical material such as lithium niobate (LiNbO3).
U.S. Pat. No. 5,384,799 describes a semiconductor gain element which is coupled to and frequency locked to surface acoustic modes generated by an acousto-optical transducer located at a front facet of a compound external cavity of a laser. Such frequency locking served only “internal” purposes to produce a highly coherent laser with narrow linewidth. As soon as laser is locked, “external” frequency modulation is not possible any more. Redfern Integrated Optics, Inc, based in Santa Clara, Calif., has developed semiconductor based external cavity lasers with integrated Silica-on-Si planar waveguide Bragg grating, commercially known as PLANEX. Such ECLs are high performance semiconductor laser sources which combines ultra-low frequency noise, and narrow linewidth (˜2-3 kHz). US patent application publication 2010/0303121 describes an example of a PLANEX-type ECL. Frequency modulation in such lasers is accomplished via direct bias current modulation applied to a gain chip. This design also serves the purpose of implementing a highly coherent frequency stabilized laser.
U.S. Pat. No. 4,485,474 uses an external cavity semiconductor laser with intracavity LiNbO3 modulator and adjustable (i.e., movable) spherical mirror as an output coupler. This design is targeted for high frequency modulation of the order of hundred MHz and gives less importance to reducing coupling losses, uniform amplitude or phase FM response, and linewidth or frequency noise. This design is suitable for high frequency modulation laser source for RF communication.
U.S. Pat. No. 6,041,071 describes a 3-section external cavity widely tunable (over a C-band wavelength) distributed Bragg Reflector (DBR) laser with grating formed on the LiNbO3 waveguide. In this design a tunable mechanism helps to select desired mode in the external cavity formed by the DBR section. Such tunable mechanism is based on the application of an external electrical field in the LiNbO3 section, which changes index of refraction due to Bragg grating formed in this section. This design addresses slow but wide wavelength tuning over a C-band but fails to procure a high frequency modulation at a specified laser wavelength. It is well known in the art that DBR sections located in the external cavity typically cannot provide low frequency noise and a narrow linewidth (of the order of 10 kHz.) To achieve narrow linewidth and low frequency noise, the grating section needs to be separated from the modulated section.
US Patent application publication 2007/0286608 describes multi-section optical FM semiconductor source based on intra-cavity FM and AM modulation. Such a design targets optical communication systems with frequency shift keying having uniform amplitude of frequency modulation with frequency range from few MHz to 10 GHz and chirp management (reshaped filter functions). Frequency modulation in such a design is accomplished by modulating the electro-absorption loss of the laser cavity. Presence of the electro-absorption section creates large changes in the phase of the electro-absorption section and at the same time increases a linewidth up to MHz range.
As described above, the PLANEX-type lasers use a gain chip for direct frequency modulation. Due to the thermal time constants of the gain chip, FM is associated with large phase delay, which limits applications of direct current modulation FM approach, specifically where optical phase locking or frequency stabilization requires a wide bandwidth (BW).
None of the existing external cavity laser architectures with modulating FM section is able to simultaneously address target parameters essential for demanding applications, such as distributed interferometric sensing, optical frequency metrology and application requiring locking (such as, atomic clock, frequency reference source, laser gyro, tunable microwave source, optical phase lock loop (OPLL) etc.) The desired performance parameters include, but are not limited to:                Wide bandwidth of frequency modulation up to 100 MHz with absence of phase delay        Both uniform response of FM amplitude and phase        Narrow linewidth (below 10 kHz)        Ultra-low frequency noise of the order of frequency noise existing in fiber laser and PLANEX-type laser        
Requirements like the absence of a significant phase delay in DC to 100 MHz bandwidth (BW) present technological challenges for known distributed feedback (DFB), DBR and external cavity semiconductor lasers using direct modulation approach. The reasons for this is that in this frequency range there are two sources of frequency modulation, namely, thermal sources and electronic (adiabatic) sources. Accordingly interplay between two mechanisms results in phase reversal at a frequency window where phase of frequency modulation is close to 180 degree. In DFB, DBR lasers such phase reversal frequency is located between 500 kHz and tens of MHz, depending on thermal constants, while in the external cavity semiconductor lasers such phase reversal frequency is around 50-200 kHz depending on the length of the cavity. Another example known in the art are a fiber lasers with narrow linewidth which accomplishes frequency modulation via piezoelectric transducer (PZT) tuning of Bragg grating. This design has very limited frequency modulation bandwidth in the order of 10-50 kHz.
Accordingly, it is necessary to implement an architecture of semiconductor ECL that achieves all or most of the desired performance parameters while being compact, consuming low power, and being easy to manufacture and package. Co-owned US patent application no. US 2010/0303121 by Alalusi et al., incorporated by reference herein in its entirety, describes a semiconductor ECL device with narrow linewidth and low noise comprising a planar lightwave circuit (PLC) and a gain chip butt coupled to each other, where a high-reflection (HR)-coated back facet of the semiconductor gain chip defined the length of the external cavity. In the present invention, a lithium niobate phase tuning element with a HR-coated back facet forms part of the external cavity, offering superior frequency modulation characteristics, as described in detail below.